216 THE CONSTITUENTS OF PLANTS. IX. 



that a better method of performing the test is to reduce 

 oxalic acid by means of a little sodium amalgam, 

 thereby forming glyoxylic acid, add a few drops of this 

 solution to that of the proteid, and then sulphuric acid. 



The usual method of estimating proteids in analysis is by 

 determining the amount of nitrogen present, and then assuming 

 that this nitrogen constitutes about 16 % of the weight of the 

 albuminoids. The percentage of nitrogen found is therefore 

 multiplied by 6*25, and the product is given as the percentage 

 of albuminoids. This only gives approximate values, since all 

 proteids do not contain exactly 16% of N. The following 

 table gives the percentage of nitrogen found in various pro- 

 teids and the factor which, multiplied by the percentage of 

 nitrogen found, will give the amount of albuminoid present": 



Substance. Percentage of nitrogen. Factor. 



Mucin ... 13-80 to 14-13 7-25 to 7'08 



Albuminates ... 13-87 7*21 



Oat proteids ... 15-85 6-31 



Egg albumen ... 15-71 to 17-85 6-37 to 5'60 

 Maize proteids ... 16'06 6-22 



Casein ... 15-41 to 16-29 6-49 to 6'13 



Serum albumen... 15-96 6-27 



Peptones ... 16-66 to 17-13 6-00 to 5-84 



Wheat proteids ... 16-80 to 18-39 5-95 to 5-44 



Flax seed proteids 17-70 to 18-78 5-65 to 5-33 



(ii.) Amides. These substances have already been defined 

 as organic acids in w^hich the OH groups have been replaced 

 by NH 2 groups. In some cases, e.g., in the case of monobasic 

 acids, the product is a true amide, while in others, e.g., when 

 the acid is polybasic, there may be a true amide, in which all 

 the acidic OH groups are replaced by an equal number of 

 NH. 2 groups, or an amic acid may be formed. For example, 

 from carbonic acid, CO(OH). 2 , there are obtainable 



(1) Carbamide (urea), CO(NH 2 ) 2 , i.e., NH 2 .CO.NH. 2 , and 



(2) Carbamic acid, NH. 2 .CO.OH. 



Under the name amides are included compounds of both 

 types. 



* Wiley, Agric. Analysis, Vol. III., p. 445. 



